The extensive knowledge of the fuel properties is a fundamental prerequisite for the design and operational optimization of the plant components of a pulverized coal firing system, in particular the milling plants.
For the characterisation of the grindability of lignite the Hardgrove Index (HGI) is also frequently used. This index was developed for bituminous coals and serves first of all for the characterisation of the crushing properties for fine grinding under compressive and crushing stresses. Therefore, the index is only regarded among experts as an auxiliary variable for the characterisation of lignites, as it has only a very limited significance for the large-scale process of the grinding under impact stresses.
In order to replace the Hardgrove Index for the characterisation of lignites by a considerably more significant index, testing equipment was developed in which the sample material is crushed by impact grinding. Such equipment (spreader disk mill) was found in 2002 at the Institute for Solid and Boundary Surface Process Engineering of the Technical University Munich. The principle of this equipment is based on the definedconditions of impact crushing with the exclusion of grinding flow effects and the quantitative determination of the crushing properties of the material under stresses comparable with a large-scale plant.
When this process, which was further developed for the needs of lignite as a fuel, was presented in public for the first time at the VGB Conference Fuel Engineering and Firing Systems in Kassel in June 2006, the response was very positive.
The present instruction sheet represents the completion of a long development work with numerous series of investigations, which were carried out at RWE Power and Alstom Power Systems in the years 2004-2008. The aim was to show a way how the material property Grindability of lignites can be determined - under defined conditions,
- with maximum reproducibility and comparability,
- under stresses comparable to a large-scale plant and
- with justifiable expenditure.
For a simple classification of lignites with different grindability an impact grindability index is introduced in analogy to the Hardgrove Index (HGI). This impact grindability index is to be used by the manufacturers and operators of lignite-fired steam generators as an index for the grindability of lignite in beater wheel, beater or fan mills.
(2009)
